1. Field of the Invention
This invention relates to a magnetic sensor which can be used for various measurements and controls in the electrical and mechanical industries.
2. Description of the Related Arts
In the construction of a known heterojunction magnetic field sensor, a two-dimensional electron gas layer (referred to as 2 DEG layer hereafter) is formed in a portion of a GaAs layer not including impurities, and is located adjacent to the heterojunction interface by placing a GaAs layer not including impurities in contact with an AlGaAs layer (spacer layer) not including impurities, and then, an AlGaAs layer including N-type impurities (carrier supply layer) is connected to the AlGaAs layer (spacer layer).
But, in the use of a magnetic sensor having the above construction, a problem arises in that a high level output cannot be obtained by the sensor even when a high electrical field is applied thereto, because when a high electrical field is applied thereto, excess carriers are generated in the N-type AlGaAs layer, which contribute to an electrical parallel conduction together with the 2 DEG carriers, and because when a high electrical field is applied thereto, the energy of the electrons in the 2 DEG layer becomes high, the average Hall mobility will be reduced and therefore the magnetic field sensitivity will be saturated.
To overcome the drawbacks mentioned above, the object of the invention is to provide a magnetic sensor having a high level output without a saturation of the sensitivity thereof even when a high electrical field is applied thereto.
To attain the object of the invention, the magnetic sensor is basically constructed as a heterojunction type magnetic field sensor composed of a heterojunction forming a two-dimensional electron gas layer having a high mobility and provided at a junction portion of at least two different kinds of semiconductor layers having a different band gap from each other, the magnetic sensor being provided that with at least a semiconductor layer having a quantum well structure in at least one of the different kinds of semiconductor layers, and the ground state subband thereof being higher than that of the two-dimensional electron gas layer.
To facilitate understanding of the principle of this invention, first the design theory for a Hall device, which is the basic magnetic sensor, is explained.
In a Hall device having a rectangular shape, the relationship between the input voltage V.sub.in and the output voltage (Hall voltage) V.sub.H is represented by the following equation, when the V.sub.in is sufficiently low. EQU V.sub.H =(W/l).multidot..mu..sub.HO .multidot.B.multidot.V.sub.in ( 1)
Wherein, l and W represent the length and the width of the Hall device, respectively, .mu..sub.HO represents the Hall mobility at the low electrical field of the device, and B represents the magnetic flux density.
When the input voltage is increased, the reduction of the Hall mobility caused by the generated Joule heat and the characteristic of the electrical field dependency must be considered, and therefore, the maximum Hall voltage V.sub.HMAX is represented by he following equation. EQU V.sub.Hmax =B.multidot.W.multidot..mu..sub.H (E).multidot.(2.multidot.h.multidot..DELTA.T.multidot..rho./t).sup.1/2( 2)
Wherein, .mu..sub.H (E) , h, .rho., and t represent the Hall mobility in a high electrical field, the coefficient of heat transfer, the resistivity, and the thickness of the Hall device, respectively, and .DELTA.T represents the difference between the temperature of the Hall device and that of the ambient atmosphere.
In this equation (2), .rho./t represents the resistance of the device, but the value of this resistance of the device preferably has a range of, for example, several ten ohms (.OMEGA.) to several kilo ohms (.OMEGA.), to match the external circuits.
Therefore, to obtain a high output from the device even when a high electric field is applied thereto, the Hall mobility thereof under such a high electric field must be made large, in accordance with the conditions mentioned above.